refactor(date): use Instant for internal representation [WPB-10090]

[MohamadJaara]

https://wearezeta.atlassian.net/browse/WPB-10090

What changed

The app now keeps dates as Instant internally instead of converting them to strings and parsing them again later.

Dates are formatted only when the UI needs text. This also removes the unused String-based date helpers and updates the related tests.

The benchmark stays opt-in, so normal instrumentation runs do not execute it.

Benchmarks

Physical Android device, 800,000 calls per path:

Formatting	Runtime: String → Instant	Speedup	Allocation/op: String → Instant	GC count/time: String → Instant
Device date	9.200 s → 6.312 s	1.46×	4,619 B → 2,850 B	16/940 ms → 9/520 ms
Medium date	5.492 s → 1.911 s	2.87×	3,658 B → 1,000 B	12/698 ms → 3/200 ms
Full date	5.521 s → 1.938 s	2.85×	3,714 B → 1,057 B	12/679 ms → 3/197 ms
Message time	2.756 s → 0.376 s	7.32×	1,897 B → 128 B	6/348 ms → 0/0 ms

The typed paths also recorded zero blocking GC events in this run. These are comparison numbers, not performance thresholds.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.67%. Comparing base (8019696) to head (221ca3f).

Files with missing lines	Patch %	Lines
...m/wire/android/mapper/MessageDateGroupingMapper.kt	92.85%	0 Missing and 2 partials ⚠️
...e/android/ui/home/conversations/model/UIMessage.kt	33.33%	2 Missing ⚠️
...kotlin/com/wire/android/util/DateAndTimeParsers.kt	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5093      +/-   ##
===========================================
- Coverage    49.70%   49.67%   -0.04%     
===========================================
  Files          660      659       -1     
  Lines        24043    23994      -49     
  Branches      3737     3722      -15     
===========================================
- Hits         11950    11918      -32     
+ Misses       10963    10962       -1     
+ Partials      1130     1114      -16     

Files with missing lines	Coverage Δ
.../wire/android/mapper/SystemMessageContentMapper.kt	46.98% <100.00%> (-0.32%)	⬇️
...otlin/com/wire/android/ui/WireActivityViewModel.kt	73.69% <100.00%> (+0.20%)	⬆️
.../android/ui/authentication/devices/model/Device.kt	93.33% <100.00%> (+6.66%)	⬆️
...ndroid/ui/home/conversations/AuthorHeaderHelper.kt	85.71% <100.00%> (ø)
...conversations/composer/MessageComposerViewModel.kt	76.53% <ø> (-0.24%)	⬇️
...kotlin/com/wire/android/util/DateAndTimeParsers.kt	72.41% <88.88%> (-0.32%)	⬇️
...m/wire/android/mapper/MessageDateGroupingMapper.kt	90.56% <92.85%> (+7.80%)	⬆️
...e/android/ui/home/conversations/model/UIMessage.kt	40.84% <33.33%> (-0.24%)	⬇️

... and 1 file with indirect coverage changes

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